1. Field of the Invention
The present invention relates to an oxide thin film transistor. More particularly, the present invention relates to an oxide thin film transistor in which a capping layer is formed on a channel region, thereby enhancing electrical characteristics, and a method of manufacturing the oxide thin film transistor.
2. Description of the Related Art
Thin Film Transistors (TFTs) are currently utilized in various application fields. In particular, TFTs have been used as switching and driving devices in the display field and as selective switches of cross-point type memory devices.
While Liquid Crystal Displays (LCDs) have been mainly used as display panels for TVs, much research into Organic Light-Emitting Displays (OLEDs) has also been conducted for their application to TVs. Development of TV display technology is being promoted to satisfy market demands. The market demands include larger-sized TVs or Digital Information Displays (DIDs), lower costs, higher-quality images (better dynamic image presentation, definition, luminosity, contrast ratio, color reproduction), etc. In order to satisfy these demands, in addition to the fabrication of large-sized substrates (e.g., glasses), high-performance TFTs that can be utilized as switching and driving devices for displays are required.
Amorphous silicon TFTs (a-Si TFTs) are used as driving and switching devices for displays. Such a-Si TFTs are devices that can be uniformly formed on large-sized substrates at low costs, and thus, are currently the most widely used devices. However, with a recent trend towards large-size and high image quality displays, TFTs are required to have a high performance. However, it is thought that conventional a-Si TFTs with a mobility of 0.5 cm2/Vs have reached a limit in their application. In this regard, high-performance TFTs with a higher mobility than conventional a-Si TFTs and their fabrication technologies are needed.
Polycrystalline silicon TFTs (poly-Si TFTs), having outstandingly better performance than conventional a-Si TFTs, have a high mobility of several tens to several hundreds of cm2/Vs, and thus, the poly-Si TFTs can be applied in displays with high image quality, which is a fact that cannot be realized by conventional a-Si TFTs. Moreover, the characteristics of poly-Si TFTs hardly degrade in comparison to conventional a-Si TFTs. However, in order to manufacture poly-Si TFTs, complicated processes are required as compared to conventional a-Si TFTs, thus incurring additional costs. That is, poly-Si TFTs are suitable for manufacturing displays with high image quality, and thus, can be applied to products such as OLEDs, however, poly-Si TFTs are less cost-effective than conventional a-Si TFTs, and thus, are restrictively applied. In addition, with respect to poly-Si TFTs, due to technological problems such as the limitation in the manufacture equipment or lack of uniformity, the formation of poly-Si TFTs on large-sized substrates has not been currently realized, which makes it difficult to apply poly-Si TFTs to large-sized TV products.
Thus, a new TFT technology for TFTs having both the advantages of a-Si TFTs and poly-Si TFTs is required, and research on this is being actively carried out. Thus, oxide semiconductor devices are a representative example of this.
Specifically, ZnO-based TFTs have been recently highlighted as such oxide semiconductor devices. ZnO, InZnO, IZO, GaInZnO, GIZO, etc. are currently known as ZnO-based materials. ZnO-based semiconductor devices can be manufactured from an amorphous ZnO-based semiconductor using a low-temperature process, thus enabling easy manufacture of ZnO-based semiconductor devices on large-sized substrates. A ZnO-based semiconductor is a material with high mobility and has excellent electrical properties like polycrystalline silicon. Currently, research is being carried out in order to apply an oxide semiconductor material layer with high mobility, i.e., a ZnO-based material layer, to a channel region of a TFT.